Fair Value Disclosure (Narrative) (Details) $ in Thousands	Jun. 30, 2016 USD ($) derivative_instrument	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt excluding capital leases fair value	$ 141,700	$ 159,800	$ 168,200
Long-term debt excluding capital leases book value	$ 141,700	159,800	168,200
Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of delayed start interest rate swaps | derivative_instrument	4
Notional amount	$ 25,000
Cash Flow Hedging | Designated as Hedging Instrument | Delayed Start Interest Rate Swap, Effective January 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of delayed start interest rate swaps | derivative_instrument	2
Notional amount	$ 15,000
Interest rate swap interest rate	1.60%
Cash Flow Hedging | Designated as Hedging Instrument | Delayed Start Interest Rate Swap, Effective January 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notional amount	$ 10,000
Interest rate swap interest rate	1.70%
Level 1 | Fair value measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mine Water Treatment Trust	$ 7,498	$ 7,247	$ 7,256
Bellaire Corporation | Level 1 | Fair value measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mine Water Treatment Trust	$ 5,000